This invention relates to nonwoven fabrics that are formed of fibers bonded randomly to one another by small discrete particles of an added polymeric material at contacts between thermoplastic fibers by heat. These nonwoven fabrics have improved kinesiological properties over prior nonwoven fabrics made of the same materials.
Nonwoven fabrics have been limited in their applications by stiffness in simple bending and resistance to deformation under complex stresses such as those encountered in bodily movements. Consequently handle or feel of nonwoven fabrics is liable to be harsh and such fabrics generally do not drape well. Achievements of softness and draping properties (without substantial loss of strength) have been major obstacles to more extensive applications of nonwoven fabrics. Nonwoven fabrics usually are formed by laying loose fibers on a moving support, hence the fibers are guided in a direction of their lay to some extent by initial contact of ends of the fibers with the moving support and this initial contact generally causes them to lie largely in the direction of movement of the support, which would be the length of the fabric. Such fabrics when supported solely by a central area thereof tend to flex along an axis that is substantially parallel to the length of the fabric, the direction in which most of the fibers are oriented.
Considerable development work has been performed to improve nonwoven fabrics so as to make them soft to the feel and to impart to them greater kinesiological properties. With particular respect to kinesiological properties, emphasis has been placed on making the crosswise strengths more compatible with lengthwise strengths (while not substantially weakening the fabrics lengthwise). Nonwoven fabrics have been produced commercially which have hand and drape properties quite similar to woven fabrics, are relatively inexpensive to make, can be sterilized and are relatively quiet when flexed whereby said nonwoven fabrics are suitable for use in expendable garments, diaper skins, skins for sanitary napkins and the like. These results have been achieved in the prior art by subjecting such nonwoven fabrics to compacting forces between faces of the fabrics, which forces crowd, flex and rearrange the fibers together lengthwise with stretches of fibers between their point of bonding to other fibers. Also these forces locally flex, crimp and bend the fibers crosswise. During such compacting, pressure also is applied to opposite faces of the fabrics adequate to prevent creping of the fabrics.
In essential features known apparatus suitable for achieving the desired compacting forces include a heated cylindrical drum mounted for rotation on its axis. A relatively thick endless belt of an elastomeric material passes between one peripheral sector of the drum and a compactor bar and thence onto the drum. The compactor bar engages the belt approximately radially of the drum until a suitable nip between the belt and the drum is established. The belt on or near its surface that engages the bar has a cord reinforcement that restricts lengthwise stretching of the belt along the reinforced face, but permits local stretching of the other face that abuts the drum. The nonwoven fabric is fed through the nip between the belt and the drum thence onto the drum for some distance around its periphery and then the nonwoven fabric is removed from contact with both the drum and the belt.
In passing through the nip between the belt and the drum, the nonwoven fabric adheres frictionally to the stretched surface of the belt passing the nip, and as the elastic belt surface unstretches in passing onto the drum, the fabric is compacted lengthwise. Because pressure on the fabric (which is substantially normal to its faces) restricts the fabric from becoming materially thicker due to its lengthwise compaction, the fibers are crowded together crosswise, rearranged and crimped locally all within the space between the faces of the fabric. This compaction causes more of the fibers to be oriented crosswise and increases crosswise tensile strength of the fabric.
In implementing such compacting on a commercial scale, it has been found that for some nonwoven fabrics hand and drape improvements require relatively high temperatures of operation of the drum in the order of 160.degree. to 280.degree. F. (71.degree. to 138.degree. C.). When drum temperatures are not sufficiently high, some nonwoven fabrics are found not to compact satisfactorily. However with high temperatures, the fabrics can stick to the drum resulting in damage to the fabrics and/or poor or no compaction. This sticking is especially troublesome in nonwoven fabrics which contain heat sealable thermoplastic binders such as are used generally in diaper skins, skins for sanitary napkins and the like. Steam sprays have been used to provide lubrication on the drum surface, but such steam sprays have not proven to be completely satisfactory. Desirable high operating temperatures have been achieved by use of release agent such as silicone oils and the like, but these release agents are expensive, they inhibit heat sealing properties of polymeric materials and they may be quite unacceptable in the resulting nonwoven fabric.